Buttons F7, F8, F9 for sound not working

How can I get these F8 + F9 sound buttons to work properly? I've had this XPS 15 L501x just a few weeks and I already screwed it up. Have tried everything to get them to work and I'm sure it's a simple fix. I just hate to call dell support.

How can I get these F8 + F9 sound buttons to work properly? I've had this XPS 15 L501x just a few weeks and I already screwed it up. Have tried everything to get them to work and I'm sure it's a simple fix. I just hate to call dell support.

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have you checked if perhaps the function keys are performing the multimedia/dispay settings or the actual "f1" etc settings?
if you press the touch sensitive gears button or open windows mobility center some options will come up and you can chose the default functions these keys perform, or you can even change it in the bios.

perhaps you've tried this already but its the only thing i can think of >.<